Why was anybody expecting the Girls aloud album to be huge? Its essentially just another Greatest hits collection. Their previous greatest hits album went triple platinum in the UK and sold over 1 million. People aint gonna buy the same thing all over again just because of a few new songs tagged on that they could DL anyways.
The single is doing badly though. But they've been away too long and pure pop has a young fickle market. Take that crossed over into adult contemporary pop so they latched onto an older market when they came back.
But GA are only back to do a farewell tour so I doubt they care too much about chart positions at this stage. The tour is a sell out and thats really what matters.